1. Field of the Invention
This invention relates to a process and a manufacture for welding bonding and/or substrate fusing surfaces together and in particular to welding using a thermoplastic as the weld material and a conductor, especially a non-metallic conductor, to heat the thermoplastic using resistance or induction heating.
2. Background
Polymeric adhesives have been commonly used to bond various substrate surfaces together. However, such materials often involve the use of monomers, catalysts, solvents and other components that present environmental and health hazards. In the case of thermoplastic composite substrates, such adhesives often fail to match the thermal performance of the composite materials they bond. Furthermore such adhesives often have relatively long curing times. Such curing times can be hastened by heating. However, heating lamps, ovens, autoclaves and the like are often difficult to use, are expensive, and can result in excessive heating resulting in deformation, uneven heat distribution and decomposition.
When the materials to be bonded are thermoplastic polymer composites, they can be joined by simply heating the interface by means of a heating element or hot plate to a temperature sufficient to allow them to be forged together when the heating source is removed. Similarly, the interface can be heated radiantly. However, these techniques have the disadvantage of being limited to the joining of small structures, the physical size being limited by the size of the hot plates, infrared lamps, and other similar heat sources. Furthermore, the bond line is often located in positions so to be inaccessible to such heating devices. In addition to the above limitations, delamination of the composite can be a serious problem.
In an attempt to solve the heating problem, Beck (U.S. Pat. No. 2,742,390) and Thompson et al (U.S. Pat. No. 3,348,640) have demonstrated the use of metallic wire resistance heating for the purpose of heating and curing polymeric adhesives. However, this method remains complicated by vapor entrapment, insufficient wetting and adhesion to one or more substrates and incompatibility in chemical resistance and mechanical strength at higher temperatures and with aging. Further problems with bonding are accentuated with higher melting engineering resins and difficult to bond polymers because of the extensive and expensive surface treatment required.
To alleviate the monomeric component and solvent hazards and problems, Thalmann et al (U.S. Pat. No. 4,618,168) have used a metallic heating wire to heat-seal two pieces of thermoplastic conduit together by fusing them to each other and to a thermoplastic sleeve surrounding the conduit joint. Sindt (U.S. Pat. No. 4,120,712) uses an induction heating device to produce eddy-current heating in steel shims to melt a hot-melt adhesive covering the outer surface of the shim.
The use of a metallic element made from wires, films, screens and the like for direct resistance and induction heating has the disadvantage of introducing a foreign material into the thermoplastic joint. In hostile environments, corrosion and seepage in association with the metallic material can lead to deterioration and delamination of the plastic joint.
Rather than using a metallic element, Thorsrud (U.S. Pat. No. 4,707,402) uses electrically conductive particulate materials such as carbon black, metal oxides and mineral powders in a bonding film so as to provide a material with a high dielectric loss factor. When placed between two adjoining materials to be bonded and exposed to a radio-frequency, dielectric field, the bonding film is heated causing the adjoining materials to be bonded. Similarly, Heath et al (U.S. Pat. No. 4,765,859) uses a ferrous metal powder mixed with a thermoplastic bonding agent to bond a flanged tube to a filter cloth using radio-frequency, induction heating. However, metal, metal oxide, and carbon particles, because of their predominate rounded shape, do not serve to give reinforcement to the bond line. Rather, they often serve as crack initiation points that weaken the bond line.
Siewert et al (U.S. Pat. No. 4,276,109) forms seams in clothing fabric by using radio-frequencies to heat a first fiber in the cloth which causes a second fiber component to become molten and act as the bonding agent. This method is limited to small bond areas and thin films less than 0.125 cm thick. Such methods are not well suited to advanced thermoplastic composites since these materials are structurally rigid in nature and are typically used in substantial thicknesses of several centimeters.
Felix et al (U.S. Pat. No. 4,871,412) has demonstrated inductive heating for consolidated thermoplastic substrates containing layers of unidirectional electrical conductors at a frequency of 1-10 MHz. Such heating was noted to be ineffective below 400 kHz and above 27 MHz with a preferred heating range of 2-4 MHz. The method relies on bulk heating and the use of pressure rollers to effect heat transfer and effective bonding. The upper composite material is bulk heated more readily near the upper surface (the surface nearest to the induction coil) with heat flow downward through the composite to the bond line. As such, the induction heating must be applied slowly to avoid overheating of the composite surface nearest the induction coil and pressure must be applied to the substrates in order to insure good contact between the relatively cool lower substrate and the downward heat flow from the bulk heated upper composite substrate. Such bulk heating causes the composites to delaminate and the fiber layers to distort. To avoid such delamination and fiber-layer distortion, the outer surfaces of the composite and substrate must be pressurized from both sides during and/or immediately after the welding operation.
Burke (U.S. Pat. No. 4,673,450) has developed a method of resistance welding of carbon fiber laminates by applying a current perpendicular to the laminates so as to soften the thermoplastic of the laminates sufficiently to form a weld. The method is cumbersome and inadequate in that the surfaces of the laminates must be roughened to expose the carbon fibers in order to insure current conduction and, more seriously and as with Felix, this bulk heating technique can cause delamination and fiber distortion of the composite.
Because of the high resistivity of non-metallic fibers such as carbon fibers, attempts to clamp metal electrodes directly to opposite ends of the fibers in a longitudinal arrangement rather than the perpendicular electrode arrangement of Burke have met with little success. Use of fibers of more than a few inches requires ever increasing voltages that tend to char and burn the fiber resulting in lost of conductivity. Furthermore voltages of more than 100 volts are generally considered to be lethal.
As a result of these limitations, it has been difficult to obtain long, continuous welds in large structures without having to use 1) toxic solvents, 2) polymeric adhesives that often contain toxic monomeric components, 3) materials such as metallic components that are introduced into and weaken the bond line (weld), or 4) bulk heating of one or more of the substrates which causes delamination and fiber distortion of the substrate.